1. Technical Field
The solution according to one or more embodiments relates to the microelectronics field. More specifically, a solution relates to bipolar injection transistors.
2. Discussion of the Related Art
Bipolar injunction transistors (BJT), or simply bipolar transistors, are commonly used in most integrated circuits. A problem of the bipolar transistors is that they are quite sensitive to the presence of mobile charges in an insulating layer (for example, a silicon dioxide layer) that covers a front surface of a semiconductor chip wherein the bipolar transistors are integrated. Indeed, these mobile charges may cause undesired and uncontrolled variations in several electrical parameters of the bipolar transistors, such as their Direct Current (DC) forward current gain (hFE).
With reference in particular to Small Signal (SS) bipolar transistors (typically used as switches or amplifiers), the mobile charges may be due to pollution (for example, of Potassium, Sodium, Copper, or other alkaline elements) during a corresponding production process. This problem is generally addressed by providing a clean process environment that limits the above-mentioned pollution.
In any case, mobile charges may be created when the bipolar transistors are exposed to ionizing radiations. Indeed, when particles or electromagnetic waves with sufficiently high energy (such as γ-rays) strike the chip, they cause the detachment of electrons from their atoms in the insulating layer, so as to create corresponding free electron-hole pairs. Due to the high mobility of the free electrons in the insulating layer, they drift very fast and recombine at the terminals of the bipolar transistors. Conversely, the free holes have low mobility, so that they remain trapped in the insulating layer where they gradually accumulate. The corresponding positive charge in the insulating layer creates an inverting layer (with a negative charge) in P-type regions at the front surface of the chip (for example, in base regions of bipolar transistors of NPN type with vertical structure). As a result, a depletion layer between each base region and a corresponding emitter region at the front surface of the chip enlarges, thereby lowering the DC forward current gain of the bipolar transistors.
The above-mentioned problem is particularly acute in special operative conditions, such as in avionic and aero-spatial applications—wherein the bipolar transistors are typically exposed to ionizing radiations due to cosmic radiations and solar winds.
In order to cope with this problem, radiation hardening techniques have been devised to make the bipolar transistors more resistant to the ionizing radiations—with the resulting bipolar transistors that are commonly referred to as radiation-hardened (or simply rad-hard) bipolar transistors.
For example, US-A-2005/0287754 (the entire disclosure of which is herein incorporated by reference) proposes a production process wherein a window is opened through a portion of the silicon dioxide, so as to expose the whole emitter region and adjoining areas of the base region on the front surface of the chip. A thin oxide layer (for example, a gate oxide) is thermally grown in this window. A field plate is formed over the emitter region and the adjoining areas of the base region; a window is then opened through the field plate and the thin oxide layer to expose a portion of the emitter region. An emitter terminal is formed in correspondence with this window, so as to contact both the emitter region and the field plate. The thin oxide layer reduces the volume that is available for trapping the free holes, thereby reducing their accumulation. At the same time, the field plate being connected to the emitter terminal (so as to be always biased in operation to a base-emitter voltage Vbe) limits the electric field at the front surface, and it enhances the surface concentration of free holes in the base region. The same document also discloses an alternative implementation, wherein a metal layer is patterned to form a combined emitter terminal (contacting the emitter region through the window opened in the thin oxide layer) and field plate (extending on the thin oxide layer).
Alternatively, U.S. Pat. No. 5,881,111 (the entire disclosure of which is herein incorporated by reference) proposes a bipolar transistor with an emitter region of N− type, a base region of P-type inside the emitter region, and a collector region of N+ type inside the base region. An emitter contact ring of P+ type is added at the front surface, extending in part into the emitter region and in part into the base region; the contact ring has a very high impurity concentration, at least three order of magnitude greater than the one of the emitter region. This emitter ring reduces the superficial inversion of the base region due to the trapped free holes.
However, the techniques known in the art are not completely satisfactory (especially for applications having very strict radiation hardening requirements).